1. Field of the Invention
The present invention relates to a method and an apparatus for controlling a squeeze pin used for pressurizing melted metal charged into a cavity of a metal mold in a die casting machine.
2. Description of the Prior Art
In general die casting, whenever melted metal is solidified in a cavity of a metal mold, there exists such a tendency that shrinkage porosities are easily produced in casted products due to shrinkage of volume of the casted products. The shrinkage porosities of course deteriorate the quality (e.g., strength) of the casted products. To overcome this problem, therefore, a method of preventing shrinkage porosities from being produced with a squeeze pin has been so far adopted, in which the squeeze pin is pushed into melted metal in the cavity of the metal mold to locally pressurize the melted metal.
In this pressurizing method by use of the squeeze pin, it is indispensable to decide an appropriate pressure and an appropriate pressurization timing according to various casting conditions. An example of method of controlling this squeeze pin is disclosed in Japanese Published Unexamined (Kokai) Patent Application No. 4-118167, for instance. In this prior art squeeze pin control method, the melted metal is pressurized under appropriate conditions by detecting the stroke of the squeeze pin, and by controlling the timing at which the squeeze pin is pushed into the cavity so that the detected stroke of the pin can be set to a predetermined value.
In this prior art control method, however, it is necessary to previously determine the optimal stroke of the squeeze pin for each pin, according to the metal mold and the casting conditions. Therefore, in case the stroke of the squeeze pin is set erroneously, there exists a problem in that the squeeze pin is controlled on the basis of an erroneously set value.
Further, when a plurality of squeeze pins are used for one metal mold, since the respective strokes of cylinders for driving the respective squeeze pins differ from each other, it is necessary to determine and control each of the cylinder strokes (i.e., pin strokes) at an optimal value, respectively. In summary, in the prior art method of controlling the squeeze pins, since the strokes of the squeeze pins differ delicately from each other according to the squeeze pins and the casting conditions, there exits a problem in that toilsome and complicated labor has been so far needed for controlling the squeeze pins in the die casting machine.